Fleury, G. (Corresponding author)FZJ* ; Schubert, F.FZJ*

1997
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-3436

Abstract: Nickel-base superalloy blades of the first rotor stage in a gas turbine have to withstand extremely severe thermomechanical loading conditions. Single crystal blades exhibit a highly anisotropie deformation behaviour and are subjected to triaxial stress fields induced by complex cooling systems. Consequently the prediction of their deformation behaviour requires constitutive equations based on multiaxial formulations. The microstructural evolution of $\gamma / \gamma$' superalloys during the service time modifies the material properties and has therefore to be taken into account in the constitutive equations. For the modelling of the anisotropic, viscoplastic behaviour of single crystal blades taking into account the evolution of the microstructure, a microstructure-dependent, orthotropic Hills potential, whose anisotropy coefficients are connected to the edge length of the $\gamma$' -particles, is applied. The prediction was validated by investigating the deformation behaviour of the superalloy CMSX-4 in the range of temperatures [750°C - 950°C]. If the shape of $\gamma$' -particles remain cubic, for example, in creep testing at low temperatures (up to about 850 $^{\circ}$C, the microstructure-dependent potential leads to the cubic version of the Hills potential. The prediction is in good agreement with creep results for <001>- and <111>- orientated specimens but overestimates the creep resistance of <011>- orientated specimens. If the $\gamma$'-particles coalesce to form rafts, the viscoplastic response of the superalloy is continuously modified. Rafting reduces the creep resistance of <001>-orientated specimens but does not occur in <111>- orientated specimens. This leads therefore to an anisotropy change between <001>- and <111>- orientations, which is successfully simulated using the microstructure-dependent potential.

1. Institut für Werkstoffe und Verfahren der Energietechnik (IWV)